This invention relates to an optical magnetic recording member.
Recently, a new optical magnetic recording system capable of high density recording has been noted and studied. The medium used for this recording system is a thin film of MnBi or an amorphous thin film of a rare earth element- Fe, Co. This thin film must have the following two properties. First, it must be a perpendicular-type magnetic film which has a strong magnetic anisotropy in the direction of the film thickness. A perpendicular-type film as used herein is one in which the magnetic particles deposited on the film base are perpendicular to that base. Second, this film must have a large polarization rotation angle. A drawback in the thin film of the prior art recording medium is that it does not have a sufficiently large Kerr rotation angle. Thus, it is difficult to use in practice.
As a means for improving the Kerr rotation angle associated with the thin film, it has been proposed, as shown in FIG. 1, that a perpendicular-type magnetic film (B) of MnBi or TbFe of 50-200 .ANG. A in thickness, formed on a transparent substrate (A), be backed with a reflection film (C) made of Al or Cu or the like.
This new type of thin film is characterized by a larger polarization rotation angle which is the combination of a Kerr rotation with a Faraday rotation. The Kerr rotation occurs when the polarization plane is rotated when a laser ray is reflected at the front surface of the perpendicular magnetic film. The Faraday rotation occurs when the polarization plane is rotated when a laser ray penetrates the perpendicular magnetic film, is reflected at the reflection film, and transmitted back through the perpendicular magnetic film. The Faraday rotation is proportional to the transmitting distance, specifically, twice the film thickness.
One would expect that the polarization rotation angle would increase if the thickness of the perpendicular-type magnetic film is increased. However, the conventional perpendicular-type magnetic film made of a metallic film of MnBi or TbFe is characterized by large optical absorption and reflection and it is not possible to produce a film having sufficient thickness. Additionally, the faraday rotation angle (as well as the Kerr rotation angle) is essentially proportional to the saturated magnetization of the magnetic member. Thus, it is desirable that the perpendicular-type magnetic film have a large transmission factor and have high saturated magnetization.